Should I lap my 1k before use?

It's about 2 inches shorter  length and 1 inch shorter width.

Personally I would not use something narrower but that is just me. Easiest way to lap is to draw a crosshatch pattern on the stone with a pencil or sharpie and lap it until all lines disappear and the stone is level.

^^^I ended up doing just that. I drew a grid on the stone with a pencil and sanded it down with 600 grit paper.

I also ended up doing the other side so I don't get them mixed up.